People who are obese usually have high blood pressure, high cholesterol and diabetes, but some obese individuals don’t suffer from these conditions. The key to understanding this phenomenon may lie in the liver, according to a new study published by the University of Helsinki.

The Finnish researchers studied pairs of twins who were genetically identical and usually raised in similar environments. But, on average, one twin of each pair weighed 17 kilograms more than their sibling. The obese twins who had much fattier livers than their thin siblings showed other signs of ill-health, such as high cholesterol, insulin resistance, high blood pressure and inflammation. But those whose liver fat was similar to that of their thin twin lacked these symptoms. The unhealthy obese twins also had fewer but larger fat cells than their thin twins, unlike the healthy pairs.

It remains unclear why fat accumulates in the livers of some and not others, but previous studies suggest that organelles called mitochondria need to be present for new fat cells to grow. Anti-inflammatory drugs might boost the activity of mitochondria to promote the growth of healthier fat while also reducing the risk of developing diabetes.